1. Field of the Invention
The present invention relates to variable resistor, especially to a variable resistor that is free of rotation angle limitation and has regular changes in resistance value while turning this variable resistor.
2. Description of the Related Art
A variable resistor is a passive component having three terminals that is commonly used to adjust a level of analog signals, such as a volume control of a radio or a brightness control of a lamp.
With reference to FIG. 8, a conventional rotary variable resistor comprises a casing (60), a substrate (61), a resistance element (62), a shaft mount (63), a shaft (64), a wiper mount (65) and a wiper (66).
The casing (60) has a stopper (600) formed inside the casing and extends from an inner bottom of the casing (60).
The substrate (61) is mounted on the casing (60) and has a through hole (610) and multiple copper foils (611). The copper foils (611) are formed on a front surface of the substrate (61) and includes a first copper foil (611a), a second copper foil (611b) and a third copper foil (611c). The first copper foil (611a) has an annular segment surrounding the through hole (610). The second copper foil (611b) and the third copper foil (611c) are respectively formed beside the first copper foil (611a) with a gap.
The resistance element (62) is mounted on the substrate (61), surrounds the annular segment of the first copper foil (611a) and has two ends respectively connecting the second copper foil (611b) and the third copper foil (611c). A maximal and constant resistance value of the conventional variable resistor can be measured between the second and third copper foil (611b, 611c).
The shaft mount (63) is mounted on a back surface of the substrate (61) and has a bushing (630) extending into the through hole (610) of the substrate (61).
The shaft (64) is mounted through the bushing (630) of the shaft mount (63).
The wiper mount (65) is attached to an end of the shaft (64) and rotated by the shaft (64) and has a lip (650) longitudinally protruding from an edge of the wiper mount (65) and abutting against the stopper (600) of the casing (60) to limit the rotating angle of the wiper mount (65).
The wiper (66) is electrically conductive, is mounted on the wiper mount (65), rotates with the wiper mount (65) and has multiple first contacts (660) and multiple second contacts (661). The first contacts (660) contact the resistance element (62). The second contacts (661) contact the annular segment of the first copper foil (611a).
When the wiper (66) rotates with the wiper mount (65), the contacts (660, 661) then slide on the resistance element (62) and the annular segment of the first copper foil (611a) and thereby changes resistance value between the first copper foil (611a) and another copper foil (611b or 611c). The stopper (600) of the casing (60) limits a rotation angle range of the wiper (66) by stopping the lip (650) of the wiper mount (65) to prevent the first contacts (660) of the wiper (66) from contacting the copper foils (611a, 611b, 611c) and causing a short circuit.
However, since the wiper (66) cannot rotate a complete revolution, rotating the shaft (64) with an inappropriate force may easily break the attachment between the shaft (64) and the wiper mount (65).
To overcome the shortcomings, the present invention provides a variable resistor that is free of rotation angle limitation to mitigate or obviate the aforementioned problems.